This invention concerns processes for the manufacture of metal foil such as aluminum foil that is widely accepted for household use as a decorative and/or utilitarian wrapping material.
Known foil products are available in a selection of thin metal gauges ranging from 0.0059 inch gauge to about 0.00025 inch gauge, for example. These foils are commonly produced by rolling processes in which the aluminum web or strand is passed through a series of rolling mills to progressively reduce the metal gauge to achieve the desired final product thickness.
In the final two rolling operations of the usual thin foil rolling process, the penultimate pass is called the split pass wherein a single web of aluminum typically of about 0.0014 inch thickness is rolled to a thickness of about 0.0006 inch. A single web of this thickness is too thin to be rolled in the final rolling operation without incurring foil strand breakage and other problems. Therefore, two such foil webs are rolled together through the final pass in a process called pack rolling to achieve a final total thickness of about 0.0005 inch for the pack or about 0.00025 inch per web. The two webs are subsequently separated, trimmed to width and wound to individual coils.
The known foil rolling processes, and in particular the split pass and pack rolling steps, must be closely controlled to achieve the most efficient and economical production rates, to minimize waste and to ensure acceptable final product quality. These objectives are not easily achieved because they tend to impose contrary requirements on the rolling process. For example, one way to increase production rates is to use so-called gray or relatively rough work rolls in the rolling mills as it is generally known that gray rolls run faster. However, it is also known that the rolled product tends to assume the surface finish of the work roll finish. Thus, gray rolls produce a gray finish on the foil which would be unacceptable in the final product as the market generally demands a bright foil finish. Therefore, gray rolls typically are utilized only prior to pack rolling and polished rolls are utilized in the pack rolling step to provide a bright surfaced foil product. The outer foil surfaces of the pack contact the polished pack rolls and thus assume a bright finish whereas the mated surfaces of the two webs of the pack assume a matte finish during pack rolling. Here too there exists the potential for contrary demands on the production process, for it is well known that incoming gray metal makes the mill run faster. Thus, if gray metal surfaces of the pack contact the pack mill work rolls, the production rate of foil through the pack mill may be enhanced. However, the use of gray rolls in the split mill to produce gray metal for pack rolling also may result in an undesirable gray undertone becoming prominent in the surface finish of the pack rolled foil. Also, any grayness in the rolls utilized in the pack mill to enhance rolling speed will tend to diminish the specularity of the final foil product. Both undertone and specularity are parameters of foil surface brightness. As the market usually demands bright foil, it is desirable to maximize specularity and minimize undertone; however, as noted above, the means of accomplishing these ends tend to adversely impact foil production rates. Still further, it is accepted practice to grind both work rolls of the split mill to a uniform and identical finish to ensure equal grayness on the opposite sides of the foil coming from the split mill. There are known instances wherein foil having differentiated surface grayness was subject to breakage and developed very poor quality surface finish during subsequent pack rolling operations.
For these and other reasons, the prior art has been fraught with numerous limitations which have frustrated the simultaneous maximizing of foil product quality and production economy.